The present invention relates to a rotary head type magnetic recording and playback apparatus such as a digital audio tape-recorder (DAT), and more particularly relates to an improvement in a rotary head type magnetic recording and playback apparatus adapted to implement an after recording (hereinafter referred to as a "postrecording") to a recorded magnetic tape.
There is generally known a helical scan type magnetic recording and playback apparatus called a DAT constituted to sequentially record pulse code modulated audio signals on a magnetic tape by means of a plurality of rotary magnetic heads.
With DAT, PCM signals obtained by applying pulse code modulation to information to be recorded are recorded on a tape conforming to an Industry Standard. After such PCM signals are subjected to recording signal processing such as time compression, they are recorded while obliquely forming signal tracks on a magnetic tape by means of rotating two magnetic heads without a guard band being formed between tracks.
In the utilized two heads type DAT, the two heads in the recording are such that a head for A channel of plus (+) azimuth (A head) and a head for B channel of minus (-) azimuth (B head) are mounted or affixed and arranged on the rotary drum so as to form an angular interval of 180 degrees to each other. These heads are used in common for recording and playback. Each head gap width (hw) is 1.5 times the track pitch (Tp).
As shown in FIG. 1, the head for common use in recording and playback rotates in a direction indicated by an arrow FA with respect to the magnetic tape 7 transported in a direction indicated by an arrow T. Thus, recording of desired data is obliquely made on the magnetic tape 7. The signal format on the recorded track 32 is constituted as well known. This signal format includes a PCM area 32A where principally audio data is recorded, ATF (Auto Track Finding) areas 32B where signals for conducting a tracking servo are recorded, and sub code (SUB) areas 32C where time codes, etc. are recorded.
In such a DAT, postrecording is conducted as follows. lnitially, ATF signals in the ATF areas 32B recorded on the tape 7 are reproduced or played back by the recording and playback common head. Thus, tracking is carried out on the basis of the reproduced signals.
In the case of tracking at the time of ordinary playback, as indicated by the position PA in FIG. 2, the center of the head 35 traces the central portion of the track 32. At the time of postrecording, however, as indicated by the position PB in the figure so that the lower edge Q of the head 35 becomes close to the boundary line of the track 32.
In the case of subcode postrecording, only when the head 35 scans the subcode areas 32C of the track 32, the head 35 is placed in a recording mode. Thus, a desired postrecording is conducted.
As stated above, since there is used a set of recording and playback common head for respective A and B channels each having a core width larger than the track width in the conventional DAT, it is necessary for performing a postrecording to conduct an offset tracking.
However, since tracking of the DAT is carried out by using pilot signals of ATF on tracks adjacent to both sides of a track subjected to tracking, it is necessary to allow the head to be positioned slightly on both adjacent tracks. Accordingly, as shown in FIG. 2, it is difficult to provide an offset such that the lower edge Q of the head 35 is positioned on the boundary line of the track.
For this reason, a track pattern after postrecording would be a pattern deviating somewhat from an original or normal pattern. For example, in the case of PCM postrecording, a deviation would occur within the PCM areas in the track, and in the case of SUB postrecording, a deviation would occur within the SUB areas in the track. Such a deviation in the pattern might cause disturbance of tracking at the time of playback, resulting in fluctuations in the playback signal level.